Neurophysiological evidence for a traveling wave in the amphibian inner ear

In response to low-frequency sounds (less than 1.0 kilohertz) auditory nerve fibers in the treefrog, Eleutherodactylus coqui, discharge at a preferred phase of the stimulus waveform which is a linear function of the stimulus frequency. Moreover, the slopes of the phase-versus-frequency functions (equivalent to the system time delays) systematically increase as the characteristic frequency of the fibers decreases. These neurophysiological observations, coupled with the known tonotopy of the amphibian papilla suggest that a traveling wave occurs in the inner ear of frogs despite the absence of a basilar membrane. Electrical tuning may contribute to these characteristic frequency-dependent delays.